1. Field of the Invention
The present invention relates to a DRAM cell capacitor and, more particularly, to a highly reliable DRAM cell capacitor for a semiconductor storage device.
2. Description of the Prior Art
As shown in FIGS. 4(a) and 4(b), a DRAM (dynamic random access memory) used as a semiconductor storage device has a capacitor has an accumulation electrode and a counter electrode (cell plate) formed opposite to each other with a capacitor insulating film therebetween, and the accumulation electrode and the counter electrode are formed of a material, such as n-type silicon. Since no inner potential is produced between the accumulation electrode and the counter electrodes formed of the same n-type semiconductor, a voltage applied across the accumulation electrode and the counter electrode is applied directly to the capacitor insulating film between the electrodes. Since the counter electrode, in general, is grounded, 0 V is applied to the capacitor insulating film in storing "0" as shown in FIG. 4(a) and 2.0 V is applied to the capacitor insulating film in storing "1" as shown in FIG. 4(b) when the supply voltage is 2.0 V. Accordingly, a relatively high maximum voltage is applied to the capacitor insulating film, which causes a problem in the reliability of the DRAM cell capacitor employing a very thin capacitor insulating film to secure capacity. In a DRAM cell capacitor proposed to solve such a problem fixes the potential of the counter electrode at 1/2 V.sub.cc, i.e., 1.0 V, instead of grounding the counter electrode. In this DRAM cell capacitor, -1.0 V is applied to the capacitor insulating film in storing "0" and +1.0 V is applied to the same in storing "1" so that the absolute value of the maximum voltage is 1.0 V.
However, it is difficult to form a circuit for stably maintaining the potential of the counter electrode at 1/2 V.sub.cc, (1.0 V) and hence the potential of the counter electrode (cell plate) is unstable. Moreover, an increased area for circuits for the generation of 1/2 V.sub.cc is required and power consumption increases.